y=x2+2x+1/(x2+2x+3)
=(x+1）2/(x2+2x+3)
当分母一定时,分子越小越好
(x2+2x+3)=（x+1）2+2永远大于零
当（x+1）2越小越好
而X=-1时y=x2+2x+1/(x2+2x+3)=0最小